Communication method and receiving apparatus

ABSTRACT

A reception method of a multicarrier system is one in which a phase-modulated data is transmitted by using each of a plurality of subcarriers, and includes a random data generating step of generating a phase shift data randomly changed, a multiplying step for multiplying each of the received subcarriers with an output obtained in the random data generating step, and a state detection signal generating step for monitoring a state of an output obtained in the multiplying step and for generating a predetermined state detection signal when a predetermined state is detected.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a communication method applicable tothe orthogonal frequency division multiplex system (OFDM system) and areceiving apparatus for receiving a signal according to thecommunication method.

2. Description of the Related Art

For a communication method suitable for a mobile communication such as awireless telephone system or the like, a multicarrier communicationmethod called Orthogonal Frequency Division Multiplexing (OFDM system)has been proposed. This system is such that a plurality of subcarriersare arranged at a predetermined frequency interval within onetransmission band and data is scattered over the respective subcarriersto modulate them for transmission. In this case, on a transmitting side,transmitting data in the form of a time sequence isorthogonal-transformed to a multicarrier signal at a predeterminedfrequency interval by a fast Fourier transform or the like. On areceiving side, a received multicarrier signal is subjected to theinverse transform of that in transmission for obtaining received data.

The transmitted signal according to the OFDM system has an advantage inthat even if there is a multipath a good transmission characteristic isensured, so that it is particularly suitable for the mobilecommunication such as the wireless telephone system or the like.

When such multicarrier signal is received, it is difficult to detect afrequency offset of the received signal. Specifically, when themulticarrier signal in which subcarriers of a predetermined number areprovided in one transmission band, for example, is transmitted in aplurality of continuous transmission bands, it is difficult for thereception side to easily determine which range of the subcarriers fromone transmission band. Especially, in order to increase the transmissionefficiency, a frequency interval between the adjacent subcarriers isvery narrow (e.g., an interval of several kHz) in the communication ofthis kind, which makes it difficult to precisely detect the frequencyoffset.

In order to solve this disadvantage, it can be considered that thetransmission side transmits a specific symbol by using a subcarrier at aspecific position in one transmission band and the reception sidecorrects an offset of a reception frequency with reference to thesubcarrier in which the reception side receives the specific symbol.However, when such a specific symbol is transmitted, the period of datatransmission is reduced to that extent, which reduces the transmissioncapacity.

SUMMARY OF THE INVENTION

In view of such aspects, it is an object of the present invention that,when transmitting the multicarrier signal, the frame period or areference timing in a frame can simply be detected on a receiving sideeven if the synchronizing signal is not transmitted.

According to a first aspect of the present invention, a reception methodof a multicarrier system, in which a phase-modulated data is transmittedby using each of a plurality of subcarriers, includes a random datagenerating step of generating phase shift data randomly changed, amultiplying step for multiplying each of received subcarriers with anoutput obtained in the random data generating step, and a statedetection signal generating step for monitoring a state of an outputobtained in the multiplying step and for generating a predeterminedstate detection signal when a predetermined state is detected.

According to a first aspect of the present invention, a receptionapparatus for a multicarrier system in which a phase-modulated data istransmitted by using each of a plurality of subcarriers, includes ademodulation unit for demodulating a signal of a multicarrier system, arandom data generating unit for generating phase shift data randomlychanged, a multiplying unit for multiplying an output from thedemodulation unit with an output from the random data generating unit,and a state detection signal generating unit for monitoring a state ofan output from the multiplying unit and for generating a predeterminedstate detection signal when a predetermined state is detected.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram showing the structure of slotsaccording to the communication method applied to each embodiment of thepresent invention;

FIGS. 2A to 2G are explanatory diagrams showing the transmission timingaccording to the communication method applied to each embodiment of thepresent invention;

FIGS. 3A and 3B are explanatory diagrams showing the band slot accordingto the communication method applied to each embodiment of the presentinvention;

FIG. 4 is an explanatory diagram showing the transmission phaseaccording to the communication method applied to each embodiment of thepresent invention;

FIG. 5 is a block diagram showing the construction of the receivingsystem according to the first embodiment of the present invention;

FIG. 6 is an explanatory diagram showing the phase shift statesaccording to the first embodiment;

FIG. 7 is a phase characteristic diagram showing an example of the phaseshift state according to the first embodiment;

FIGS. 8A to 8C are characteristic diagram showing the phase state whenassuming that data is zero with the first embodiment;

FIGS. 9A to 9C are characteristic diagrams showing the phase state whendata is scattered with the first embodiment; and

FIG. 10 is a block diagram showing the construction of the receivingsystem according to the second embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments according to the present invention will be described belowwith reference to the accompanying drawings.

In these embodiments, the present invention is applied to the wirelesstelephone system of what is called a cellular system in which a basestation is arranged in a predetermined condition to form a service areaand communicate with a portable station (terminal device). First of all,the multicarrier transmission system applied to the embodiments will bedescribed in detail with reference to FIG. 1 to FIG. 4. A communicationsystem of the present example is made to be the orthogonal frequencydivision multiplex system (OFDM system) in which a plurality ofsubcarriers are successively arranged within a preallocated band. And aplurality of the subcarriers within one transmission band aresimultaneously utilized through one transmission path, and is furthermade to modulate collectively a plurality of the subcarriers within oneband in a band division.

A structure thereof will be explained. FIG. 1 shows how to constructslots of the transmitting signal in this example, in which figure avertical axis represents frequency and a horizontal axis representstime. In case of this example, orthogonal bases are provided by dividingthe frequency axis and the time axis in a grid shape. Particularly, onetransmission band (one band slot) consists of 150 kHz and twenty-foursubcarriers are arranged within this one transmission band of 150 kHz.These twenty-four subcarriers are successively arranged at an equalinterval of 6.25 kHz and to each subcarriers are given subcarriernumbers 0 to 23, respectively. However, really existing subcarriers aretwenty-two one from subcarriers numbers 1 to 22. Regarding thesubcarrier numbers 0 and 23 at both ends within one band slot, they aremade guard band in which no subcarrier is arranged and their powers aremade zero.

Looking over the time axis, one time slot having a time period of 200 μsis defined and at every time slot the twenty-two subcarriers aremodulated by a burst signal for transmission. A time section in whichtwenty-five time slots are arranged (i.e. 5 ms) is defined as one frame.To each time slot within this one frame is given time slot numbers 0 to24, respectively. An area shown by hatching in FIG. 1 represents onetime slot section of one band slot. Further, the time slot of the slotnumber 24 is made a time period where no data is transmitted.

Using the orthogonal bases formed by dividing the frequency axis and thetime axis in a grid shape, the multiple access in which the cell stationcommunicates simultaneously with a plurality of the portable stations(terminal devices) is carried out. In this case, as to a mode ofcoupling to each portable station, it is performed in such a manner asshown in FIGS. 2A to 2G. FIGS. 2A to 2G illustrate use of the time slotsof six portable stations (users) U0, U1, U2, . . . U5 to be coupled tothe cell station through one band slot (in practice, the band slot foruse is switched by means of a frequency hopping described below), inwhich figure a time slot denoted by R is a receiving slot and denoted byT is a transmitting slot. The cell station establishes a frame timing oftwenty-four time slot periods as shown in FIG. 2A (The last slotnumbered 24 of twenty-five prepared time slots is not used.). In thiscase, the transmitting slot and the receiving slot are arranged here touse a different band transmission, respectively.

For example, the portable station U0 shown in FIG. 2B uses the timeslots numbered 0, 6, 12, 18 within one frame as the receiving slot anduses the time slots numbered 3, 9, 15, 21 as the transmitting slot,through the respective time slots the reception or transmission of theburst signal being performed. The portable station U1 shown in FIG. 2Cuses the time slots numbered 1, 7, 13, 19 within one frame as thereceiving slot and uses the time slots numbered 4, 10, 16, 22 as thetransmitting slot. Also, the portable station U2 shown in FIG. 2D usesthe times slots numbered 2, 8, 14, 20 within one frame as the receivingslot and uses the time slots numbered 5, 11, 17, 23 as the transmittingslot. Again, the portable station U3 shown in FIG. 2E uses the timeslots numbered 3, 9, 15, 21 within one frame as the receiving slot anduses the time slots numbered 0, 6, 12, 18 as the transmitting slot.Further, the portable station U4 shown in FIG. 2F uses the time slotsnumbered 4, 10, 16, 22 within one frame as the receiving slot and usesthe time slots numbered 1, 7, 13, 22 as the transmitting slot. Finally,the portable station U5 shown in FIG. 2G uses the time slots numbered 5,11, 16, 22 within one frame as the receiving slot and uses the timeslots numbered 2, 8, 14, 20 as the transmitting slot.

According such as arrangement as is shown in FIGS. 2A to 2G, six TDMA(Time Division Multiple Access) in which the six portable stations arecoupled to one band slot can be performed. Seeing from each portablestation side, after the reception and transmission during one time slotperiod had been completed, there is a spare time for two time slotperiods (i.e., 400 μs) until the next transmission or reception beginsto be performed. Thus, using this spare time, a timing process and theprocess termed the frequency hopping are performed. That is, duringabout 200 μs before each transmitting slot T, a timing process TA inwhich a transmission timing is made to match with a timing of a signalfrom the base station side is performed. After a time passage of about200 μs, when each transmitting slot T has been completed, the frequencyhopping in which a band slot for the transmission and reception isswitched over another band slot takes place.

A plurality of the band slots are assigned to one base station. Forexample, in case of the cellular system where one cell is comprised ofone base station, if a band of 1.2 MHz is allocated to one cell, eightband slots can be assigned to one cell. Likewise, if a band of 2.4 MHzis allocated to one cell, sixteen band slots can be assigned to onecell. Also, if a band of 4.8 MHz is allocated to one cell, thirty-twoband slots can be assigned to one cell. Finally, if a band of 9.6 MHz isallocated to one cell, sixty-four band slots can be assigned to onecell. In order to use equality a plurality of the band slots assigned tothis one cell, the frequency switching process called frequency hoppingis carried out.

FIGS. 3A and 3B show an example where eight band slots are arrangedwithin one cell. At each of eight band slots prepared as shown in FIG.3A, twenty-two subcarriers are arranged as shown in FIG. 3B for datatransmission. Data is transmitted over each of the respectivesubcarriers under a predetermined phase modulation. In this example,QPSK (Quadrature Phase Shift Keying) modulation is used, in which datais transmitted as data at four points of phase position shifted in turnby π/2 of a circle on an orthogonal coordinate (a circle denoted by abroken line in FIG. 4) which are formed by laying an I component atright angles to a Q component, as is shown in FIG. 4.

Wireless communication takes place in this transmission system. However,in case of the present example, the transmitting signal is made of suchdata as each subcarrier is multiplied by respective different randomphase shift data. That is, at each of the twenty-two subcarriers fromthe subcarrier number 1 to the subcarrier number 22 within one bandslot, an initial phase shift value of the first data of the first timeslot of each frame is established and then the phase shift value ischanged from the initial phase shift value in a sequence determined atrandom sequence which is determined in advance).

Next, a construction of a first embodiment according to the presentinvention in which a terminal device (portable station) receives thesignal transmitted from the base station in this way, will be describedwith reference to FIG. 5. FIG. 5 shows a receiving system of theterminal device, in which a received signal (the signal which issubjected to a receiving process in a filter or an amplifier afterreceived by an antenna) available at an input terminal 11 is supplied tomixer 12, where it is mixed with a frequency signal output by afrequency signal generator means 13 comprised of a PLL circuit (phaselocked loop circuit) for frequency converting the received signal of apredetermined transmission band (band slot) into an intermediatefrequency signal. In this case, this frequency signal generator means 13has basically an output frequency which varies at an interval of oneband slot (i.e. here at a interval of 150 kHz).

Subsequently, the intermediate frequency signal output by the mixer 12is supplied to an analogue/digital converter 14, where it is sampled ata predetermined sampling period. The received data sampled in theanalogue/digital converter 14 is supplied to window multiplier circuit15, wherein it is multiplied by window multiplying data (temporal waveform) corresponding to a window multiplying data by which it wasmultiplied on the transmitting side.

The received data multiplied by the window multiplying data is suppliedto an inverse fast Fourier transform circuit (IFFT circuit) 16, whereinit is subjected to a transforming process between a frequency domain andtime domain by the inverse fast Fourier transform operation, therebycausing the data which modulated the twenty-two subcarriers at aninterval of 6.25 kHz for transmission to be a single sequence ofcontinuous data in the time axis.

The received data transformed into a single sequence is supplied to amultiplier 17, wherein it is multiplied by random phase shift dataoutput by a random phase shift data generator circuit 18. The randomphase shift data output by the random phase shift data generator circuit18 is generated on the basis of pattern data stored in a random phaseshift pattern storing memory 19. The random phase shift data is data forrestoring the data whose phase was shifted and scattered on everysubcarrier in transmission from the transmitting side (base station) tothe original data. As to the random phase shift data, an initial phaseshift value to each subcarrier is determined and its value is arrangedto change the phase to be shifted from the initial phase position atrandom (The order in which the phase shift amount is varied at randomfrom the initial phase shift value is the same as that set up on thetransmitting side). A timing to generate the random phase shift data bythe generator circuit 18 is set up under the control of a controller 25which controls the receiving operation of this terminal device.

Thereafter, the data whose phase has been restored to original phase issupplied to a decoder 20, where the phase modulated data is decoded bythe differential demodulation or the like. The decoded data is suppliedto a four frame deinterleave buffer 21, where interleaved data over fourframes in transmission is restored to data of the original sequence. Thedeinterleaved data is supplied to a Viterbi decoder circuit 22 forViterbi decoding. The Viterbi decoded data is then supplied as thereceived data to a received data processing circuit (not shown) atsubsequent stage from a received data output terminal 23.

Moreover, in this embodiment, the decoded data by the decoder 20 issupplied to a decision circuit 24 which decides whether or not the datahas correctly been decoded and supplies the decision data to thecontroller 25. Regarding this decision, for example, when accumulatingphases of the signal over a time period in which data of the twenty-twosubcarriers forming one band slot time period is acquired, if theaccumulated phases vary at an angular interval of about π/2, thedecision data is produced that indicates that the decoding has beencompleted correctly. If the accumulated phases do not vary at theangular interval of about π/2, the decision data is produced thatindicates that it is erroneously processed data.

The controller 25 estimates, if it receives the decision data indicatingthat the decoding has been completed correctly, that there is nofrequency offset in the received signal at that time. If it receives thedecision data indicating that the decoding has been performedincorrectly, it estimates that there is a frequency offset in thereceived signal at that time. On the basis of a result of the decision,the controller 25 outputs the frequency offset correction data D₁ andmakes a fine adjustment of a received frequency at intervals of 6.25kHz.

The fine adjustment of the received frequency can be implemented by suchprocessings as changing the output frequency of the frequency signalgenerator means 13 at an interval of 6.25 kHz, or as establishing thenumber of transform points in the inverse fast Fourier transform by IFFTcircuit 16 more than the number of the subcarriers (twenty-twosubcarriers) forming one band slot and then changing a position of thetwenty-two subcarriers extracted from the transform points as a sequenceof data or the like. Furthermore, the fine adjustment of the receivedfrequency may be performed by other processing.

In addition, the controller 25 in the present embodiment outputs a frameposition detecting data D₂ based on a timing when a predeterminedcondition has been decided by the decision circuit 24. On the basis ofthe frame position detecting data D₂, a processing timing at each of thecircuits within the terminal device is set for the timing synchronouswith the received data.

Next, an operation when receiving with the terminal device according tothe present embodiment will be described. For example, when the basestation transmits, the initial phase shift amount by which the firstdata of each frame is multiplied is made a value which is determined forevery subcarrier and the phase shift amount for each subcarrier isarranged to change from the initial phase shift amount at apredetermined random manner.

FIG. 6 shows an example of the initial phase shift amounts of respectivesubcarriers. Phase shift value of transmitting data therein shows theinitial phase shift amount of each of the subcarriers in each frame. Itsvalue is determined in turn from the carrier number 0. These phase shiftvalues are here set up within a range from -π to π so that all of themwill be different phase shift amounts at respective subcarriers withinone band slot. FIG. 7 is a diagram which shows the initial phase shiftvalues of the respective carrier numbers by its phase state, in whichfigure the phases of the carrier numbers are represented as #0, #1, #2and so on.

These initial phase shift values are also stored in the random phaseshift pattern storing memory 19 of the receiving side (terminal device).On the basis of data read out of the memory 19, a processing to restorethe phases of the subcarriers by the amount phase shifted by themultiplier 17 is performed. Case 1, case 2 and case 3 denoted in FIG. 6show setting examples of the initial phase shift values by which thereceived signal is multiplied on the receiving side. In each case, theinitial phase shift values are shifted in turn one by one subcarrier.Here, for example, the receiving process for the initial phase shiftamount set up in each case is performed at every one frame. For example,in the first frame the initial phase shift amounts are set up accordingto case 1 and in the following frame the initial phase shift amounts areset up according to case 2 where they are shifted by one subcarrier. Inthe further following frame, the initial phase shift amounts are set upaccording to case 3 where they are shifted further by one subcarrier.

In the example of FIG. 6, since the initial phase shift amounts in case2 correspond with those on the transmitting side, in a frame which setsup the initial phase shift amount according to case 2 for the receivingprocess, data can correctly be decoded and correct received symbol datacan be obtained. That is, assuming that the transmitting data which istransmitted through the twenty-two subcarriers forming one band slot arethe same data of all zero data, when the initial phase shift of case 1is set up for the receiving process, the phase decoded by the decoder 20will scatter into respective phase states as shown in FIG. 8A. Also,when the initial phase shift of case 3 is set up for the receivingprocess, the phase will scatter into different phase states as shown inFIG. 8C. In contrast to this, when the initial phase shift of case 2 isset up for the receiving process, the phase will go into the same phasestate as shown in FIG. 8B. This means that the condition in which thereceiving frequency is set up according to case 2 is the condition inwhich one band slot is correctly received and there is no frequencyoffset.

In fact, the transmitting data is scattered values and so the phasestates corresponding to cases 1, 2 and 3 will change as shown in FIGS.9A, 9B and 9C. When the accumulated phases in the decision circuit 24correspond to case 2, it is decided that they change at an angular unitof about π/2 and the decision data indicating that the correct decodinghas been performed is then supplied to the controller 25. In thereceiving process corresponding to case 1 and case 3, since a change ofthe accumulated phases is not the change at an angular unit of aboutπ/2, the decision data indicating that the data has been erroneouslyprocessed is supplied to the controller 25.

Therefore, the controller 25 outputs the frequency offset correctiondata D, and makes the initial phase shift to change in turn in order tocontrol the receiving frequency to be set up correctly, until the stateof case 2 is detected.

By performing a receiving process in such a manner as the presentembodiment, when a multicarrier signal in which the subcarriers aresuccessively arranged at a narrow frequency interval is to be received,it is possible to simply detect the correct receiving coverage of oneband slot and to correct the frequency offset by a simple processing ina short period of time. In this case, since there is no need to transmita particular symbol for detecting the frequency offset as in the past,the data transmission capacity for practical use increases by that lessneed, thereby allowing a correct receiving process to be performedwithout lowering the transmission efficiency.

Also, by performing the receiving process according to the presentembodiment, it is possible to detect not only the frequency offset butalso the frame position of the transmitting signal. Further, in thiscase, it is necessary for the random phase shift pattern to bedetermined beforehand so that a different pattern may be used dependingon "the slot number in a frame given to the relevant slot. "If such anarrangement is made in advance, when, after the adjustment of thefrequency offset has been completed, for example, "the random phaseshift pattern used for the first slot in a frame" is produced by therandom phase shift data generator means 18 and then the output of thedecoder of received signal is multiplied by that random phase shiftpattern, if it is decided that the phases of the respective subcarriersvary at an angular unit of about π/2, then it can be determined that theslot is the first slot in the frame. The timing of the frame can bedetected by processing in this way.

In the above-mentioned processing, a slot signal in a certain slot isstored in a memory 31 and this signal is multiplied by the first shiftpattern of the frame, by the second shift pattern of the frame, by thethird shift pattern of the frame, etc. Thus, results of themultiplication by the respective shift patterns are decided, therebyalso allowing the slot number in the frame given to the relevant slot tobe estimated.

Next, a second embodiment according to the present invention will bedescribed with reference to FIG. 10. The construction shown in FIG. 10is that of the receiving system in the terminal device for receiving themulticarrier signal according to the system described above referring toFIG. 1 to FIG. 3, in which the corresponding parts to FIG. 5 describedon the first embodiment are denoted by the same reference numerals andare not described in detail.

In this embodiment, received data transformed by the inverse fastFourier transform circuit (IFFT circuit) 16 is supplied to themultiplier 17 which multiplies the received data by the phase shift dataoutput by the random phase shift data generator circuit 18. At the sametime, the received data transformed by the IFFT circuit is supplied to amemory 31 and after it is once stored, it is supplied to a multiplier 32which multiplies it by the phase shift data output from the random phaseshift data generator circuit 18. At this time, a timing of outputtingthe phrase shift data from the random phase shift data generator circuit18 to the multiplier 32 is made to synchronize with a timing of readingthe received data out of the memory 31. Moreover, for the memory 31, amemory of a capacity which is capable of storing the received data for apredetermined time period (e.g. one frame period or one slot timeperiod) is used. A timing of reading the data out of the memory 31 iscontrolled by a controller 35.

A multiplied output of the multiplier 32 is decoded by a decoder 33.This decoder 33 has the same structure as that of the decoder 20 whichdecodes the output of the multiplier 17. The data decoded by the decoder33 is supplied to a decision circuit 34 which decides if the data iscorrectly decoded and its decision data is then supplied to thecontroller 35. Regarding this decision, for example, when accumulatingphases of the signal over a time period in which data of the twenty-twosubcarriers forming one band slot time period is acquired, if theaccumulated phases vary at the angular of about π/2, the decision datais made what indicates that the decoding has been completed correctly.If the accumulated phases do not vary at the angular unit of about π/2,the decision data is produced that indicates that it is erroneouslyprocessed data.

The controller 35 estimates, if it receives the decision data indicatingthat the correct decoding has been completed, that there is no frequencyoffset in the received signal at that time. When the decision dataindicating that the correct decoding can not be performed, it is decidedthat there is a frequency offset in the received data at that time. Onthe basis of a result of the decision, the controller 35 outputs thefrequency offset correction data D₁ and makes a fine adjustment of areceived frequency at an interval of 6.25 kHz.

Also, the controller 35 outputs a frame position detecting data D2 basedon the timing when the predetermined state is decided in the decisioncircuit 24. On the basis of the frame position detecting data D₂, thetiming of processing in each circuit of the terminal device is made to atiming synchronized with the received data.

Concerning other parts, they are constructed in the same way as those inthe receiving system of the first embodiment shown in FIG. 5.

According to the construction of the second embodiment, by using thereceived data once stored in the memory 31, it is possible to performprocessings for the received frequency offset detection and the frameposition detection irrespective of the actual receiving condition. Inother words, if the minimum unit of data required for detecting thefrequency offset and the frame position is stored in the memory 31, itis possible that the stored data is repeatedly read out and at everyreadout time the initial phase shift amount is shifted in turn as case1, 2 and 3 shown in FIG. 6, the result of decoding by the decoder 33being decided in the decision circuit 34 for detecting the frequencyoffset and the frame position. Thus, using the received data stored inthe memory 31, it is possible to quickly detect the frequency offset andthe frame position.

Further, in the above embodiments, the processing where a signaltransmitted from the cell station is received at the terminal device hasbeen described, the present invention is of course applicable to a casewhere a signal transmitted from the terminal device is received at thecell station. In this case, the detected offset value is reported to theterminal apparatus with which the base station is communicating, and theterminal apparatus adjusts the frequency offset of a signal from itselfin accordance with the report.

Moreover, in the above embodiments, the phase shift amount is set up atrandom within the range from -π to π, whereas the phase shift amount maybe set up at random within the other range. For example, it may be setup at random within the range from 0 to π/2.

Also, in each of the aforesaid embodiments, the present invention isapplied to the multicarrier signal transmission system describedreferring to FIG. 1 to FIG. 4. However, the present invention is alsoapplicable to a communication method and its receiving apparatus towhich various transmission systems of other signals are applied. As tothe specific modulating method and encoding method, they are not limitedto those of the embodiments described above. In addition, regarding themode of communication to be applied, it is applicable to the case wherethe multicarrier signal is transmitted in various wirelesscommunications other than the wireless telephone system described above.

According to the communication method of the present invention, since aninitial phase of each of subcarriers forming multicarrier signal is setto a predetermined random value, it is possible to precisely receive thetransmitted multicarrier signal by detecting the random initial phase onthe reception side.

Since the frequency offset between the transmission band and thereception band of the reception signal is detected by detecting aninitial phase shift amount of each of subcarriers. It is possible toprecisely correct the frequency offset.

When the phase change among the respective subcarriers of the receivedsignal is in a predetermined state, it is determined that there is nofrequency offset between the transmission band and the reception band ofthe reception signal. Therefore, it is possible to easily detect thefrequency offset.

According to the reception apparatus of the present invention, since thefrequency offset between the transmission band and the reception band ofthe reception signal is determined based on the phase change between thesubcarriers determined by the determining means, it is possible todetect the frequency offset of the multicarrier signal with a simplearrangement.

In this case, the phase shift data based on the initial phase previouslyset for each of the subcarriers is multiplied with each of thesubcarriers of the received signal, and the multiplied signal is decodedby the determining means. Then, it is determined whether or not thephase change falls in a predetermined state. Therefore, it is possibleto easily determine whether or not the frequency is offset, only bydetermining the result of the decoding processing.

Since the multiplying means successively shifts the arrangement of thephase data to be multiplied with each of the subcarriers and a positionwhere the determining means determines a predetermined state isdetermined, it is possible to easily detect a reception position with nofrequency offset.

Having described preferred embodiments of the present invention withreference to the accompanying drawings, it is to be understood that thepresent invention is not limited to the above-mentioned embodiments andthat various changes and modifications can be effected therein by oneskilled in the art without departing from the spirit or scope of thepresent invention as defined in the appended claims.

What is claimed is:
 1. A reception method of a multicarrier system inwhich phase-modulated data is transmitted by using each of a pluralityof subcarriers, comprising:a receiving step for receiving the pluralityof subcarriers that are transmitted; a converting step for convertingthe received plurality of subcarriers to respective digital subcarriers;a random data generating step for generating randomly changed phaseshift data; a multiplying step for multiplying each one of the pluralityof digital subcarriers with an output obtained in said random datagenerating step; a decoding step for decoding an output of themultiplying step; a state decision signal generating step for monitoringa state of an output obtained in said decoding step and for generating apredetermined state decision signal when a predetermined state isdetected; and a detecting step for detecting a frequency offset of eachof said plurality of subcarriers based on an output obtained in saidstate decision signal generating step.
 2. The reception method accordingto claim 1, wherein said predetermined state and said predeterminedstate decision signal are defined such that when an output obtained insaid decoding step is changed by a unit of a predetermined value, it isregarded that there is no frequency offset and, in other cases, it isregarded that there is a frequency offset.
 3. The reception methodaccording to claim 1, wherein said predetermined value is π/2.
 4. Thereception method according to claim 1, comprising the further step ofdelaying each of said received subcarrier signals input in saidmultiplying step by a predetermined time.
 5. A reception apparatus for amulticarrier system in which phase-modulated data is transmitted byusing each of a plurality of subcarriers, comprising:a demodulation unitfor demodulating a received signal of the multicarrier system; a randomdata generating unit for generating randomly changed phase shift data; amultiplying unit for multiplying an output from said demodulation unitwith an output from said random data generating unit and producing anoutput signal; a decoder for decoding the output signal from saidmultiplying unit; a state decision signal generating unit for monitoringa state of an output from said decoder and for generating apredetermined state decision signal when a predetermined state isdetected; and a controller, wherein a frequency offset of the receivedsignal is detected and corrected based on an output from said statedecision signal generating unit.
 6. The reception apparatus according toclaim 5, wherein said predetermined state and said predetermined statedecision signal are defined such that when an output obtained from saiddecoder is changed by a unit of a predetermined value, it is regardedthat there is no frequency offset and, in other cases, it is regardedthat there is a frequency offset.
 7. The reception apparatus accordingto claim 6, wherein said predetermined value is π/2.
 8. The receptionapparatus according to claim 5, further comprising:a signal delayingunit for delaying an output from said demodulation unit by apredetermined time and supplying the delayed output to said multiplyingunit.